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This chapter considers carbon and energy balances within cities and between cities and nature, and the question of the long-term sustainability of these quantities. Americans use a lot of energy, dominated a century ago by coal, then overtaken by petroleum and natural gas, and now, once again, dominated by coal. Even longer ago, trees supplied most of the energy, and recent hopes have pinned the future on biomass fuels. Certainly, modern-carbon energy has advantages over fossil-carbon energy, but our total energy use far exceeds any hopes for a substantial biomass solution. Photosynthesis...

This chapter considers carbon and energy balances within cities and between cities and nature, and the question of the long-term sustainability of these quantities. Americans use a lot of energy, dominated a century ago by coal, then overtaken by petroleum and natural gas, and now, once again, dominated by coal. Even longer ago, trees supplied most of the energy, and recent hopes have pinned the future on biomass fuels. Certainly, modern-carbon energy has advantages over fossil-carbon energy, but our total energy use far exceeds any hopes for a substantial biomass solution. Photosynthesis makes the link between energy use, carbon emissions, and growing vegetation, and, of course, it provided the fossil fuels people use today. The chapter overviews how photosynthesis strongly connects atmospheric carbon dioxide (CO2), water, and light, providing the important context for sustainability calculations. Following on from the examination of sunlight and impervious surfaces producing urban heat islands, the chapter demonstrates how light interacts with trees as part of fixing carbon and shading the ground.